Queries regarding on the transconductance of the device and current buffer

[narayani]

I have following queries

(1) What is the trans conductance of the device (transistor) and what is importance and how it is effecting on the noise figure while designing microwave low noise amplifier

(2) What is current buffer, what is its use when current gain of the transistor is equal to one(1)while designing microwave low noise amplifier

You can see below transistors are in cascode fashion. Upper transistor current gain is one and when we have cascode transistors, transconductance can be increased, how is it, can you justify it

 

[saqib.shah06]

(1) The transconductance is a measure of how much output current you get for some change in the input voltage. E.g. lets say that you change the input by 1 mV, on a FEt of transconductance 1mS. The the output current (the current through the drain of the FET) will simply be 1mV x 1mS = 1uA. The noise that a transistor produces is directly related to the transconductance (refer to any standard textbook for more on this).

(2) The main use of a current buffer is to provide a low impedance source for the input current. In this circuit you have shown, the use of a current buffer is for a slightly different reason - in a cascode, the output resistance is greatly enhanced due to the use of the current buffer. Its better to think of the cascoded transistor as a Common Gate amplifier rather than a current buffer.

 

[D.A.(Tony)Stewart]

the lower FET is provide high impedance gain of RL/R or impedance reduction to drive load RL, while the upper allows full bandwidth at unity gain for modulation.

Thermal noise increases with power loss so higher transconductance reduces noise and losses.